The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Complex electronic systems employing integrated circuits are now manufacturable at sufficiently low cost that they are widely used in distributed remote sensing and various communications systems. With this development has come an increasing challenge in tracking and/or recovering such devices from the environment in which they are being used. As such, these devices may be left in the environment after their functionality has been destroyed or impaired. Accordingly, there is an increasing interest in developing electronic systems, and particularly integrated circuits, which can be destroyed upon a triggering event, and such that little or no trace of the physical materials that were used to form the circuit are left behind after the circuit is destroyed.
One important objective relating to the above-mentioned goal is developing and establishing a base set of materials, components, integration, and manufacturing capabilities to undergird this new class of electronics. An important overall objective is to develop fully transient electronic systems with performance equivalent to state of the art electronics manufactured for applications such as present day smartphones, personal computers, computing tablets, and other high performance electronics. The approach should leverage present day semiconductor technologies used to manufacture high performance electronics such as the Intel Pentium® microprocessor. The co-inventors of the subject matter of the present application are not aware of any transient electronics available which can match the performance available from the commercial microelectronics industry.
It is also important to recognize that interest in the field of transient electronics has grown over the past several years, with the most notable recent advancements being developed by Dr. John Rogers at the University of Illinois at Urbana Champaign. His Science paper in 2012 titled “A Physically Transient Form of Silicon Electronics” (link: http://www.sciencemag.org/content/337/6102/1640) and his more recent paper in Advanced Materials titled “Materials for Bioresorbable Radio Frequency Electronics” (link: http://onlinelibrary.wiley.com/doi/10.1002/adma.201300920/abstract) increased the interest and momentum in this field. In his approach, Dr. Rogers has developed electronic components which are dissolvable in water. Using ultra-thin silicon (˜35 nm), silicon dioxide, magnesium, magnesium oxide, and synthetic silk as a substrate, Dr. Rogers has demonstrated basic electronic functions performed by components such as resistors, diodes, and a RF antenna which are inherently transient. Using these components, Dr. Rogers has demonstrated simple electronic systems and components. However, the performance of these systems and components is significantly below the electronics which are able to be manufactured by present day semiconductor processes.
There has also been significant interest in the medical community and the commercial sector for circuits that can essentially “disappear” over time or by activation from a suitable signal or command. High-performance, organic, thin-film transistor arrays have been fabricated on paper which offer a compostable, low cost, disposable platform. Transient electronics can be applied to implantable biomedical devices or for the protection of sensitive property. There are several FDA-approved biodegradable polymers, such as polylactic acid and polycaprolactone, which are used to contain and package implantable electronics. Other biodegradable and biocompatible polymers include copolymers of polyglycolic acid and polyanhydrides. Silk has become a popular substrate of choice for thin-film bioelectronics due to its biocompatibility, solubility and amenability to functionalization. For conductive components, biodegradable metals such as magnesium and iron have been demonstrated in stent applications. Wireless RF pressure sensors made with biodegradable polymer and zinc have been microfabricated using MEMS technologies. As interest in more complex transient circuits increases, more research is being placed in the manufacturing and material sets required. As pointed out by Dr. Rogers and others in the field, silicon is inherently a transient material. Its rate of dissolution depends on specific environmental conditions. For example, silicon is etched extremely slowly in water (e.g., a few nanometers per day at room temperature) and much more quickly in other aqueous media such as potassium hydroxide (micrometers per minute at room temperature). However, both of these approaches have several disadvantages which preclude them being used on typical silicon microelectronics. Typical packaged silicon microelectronics are approximately 250-700 micrometers thick and contain many different materials such as silicon, silicon dioxide, silicon nitride, aluminum, gold, copper, tungsten and various polymers. A typical cross section of an NMOS transistor is shown in FIG. 1. As shown in FIG. 1, a majority of the transistor is silicon including the polysilicon for the first layer of transistor interconnects. A field oxide which is thermally grown silicon dioxide passivation (˜100 nm to 500 nm) passivate the devices. On top of the field oxide is a number of layers used to interconnect the devices. These interconnect layers consist of a passivation layer and metal layers. The passivation layer is typically silicon dioxide and the metal layer is typically copper or aluminum.
Under typically conditions, the above mentioned aqueous solutions do not dissolve all of these materials, and the materials which do dissolve do not do so in a reasonable length of time. More importantly, a reservoir is required to store the aqueous solution. The use of a fluid reservoir is not practical in many, if not most, instances. Furthermore, the use of a reservoir is easily defeatable.